The main physiological role of white adipose tissue (WAT) is to supply energy when it is needed by other tissues. In mammals, white adipose tissue is the primary energy storage depot, accumulating fuel reserves in the form of triacylglycerol (TAG) during times of energy excess (Wang M. et al., Chem. Biol., 2006, 13, 1019-1027; Gregoire F. M. et al., Physiol. Rev., 1998, 78, 783-809). However, unlike TAG synthesis that also occurs at high levels in liver for very low density lipoprotein (VLDL) production, lipolysis for the provision of fatty acids as an energy source for use by other organs is unique to adipocytes. The release of free fatty acids (FFA) from TAG proceeds in an orderly and regulated manner (Unger R. H, Annu. Rev. Med. 2002, 53, 319-336; Duncan R. E. et al, 2007, Annu Rev Nutr, 27, 79-101; Jaworski K. Et al, 2007, Am J Physiol Gastrointest Liver Physiol, 293, G1-4), stimulated by catecholamines and regulated by hormones such as insulin, glucagon and epinephrine.
The most important enzyme in WAT believed responsible for hormone regulated hydrolysis of triglyceride is hormone sensitive lipase (HSL). This enzyme is also present in the liver, skeletal muscle, pancreas and adrenal glands. In the basal state, it has minimal activity against its substrate. Stimulation of adipocytes by hormones activates protein kinase A resulting in the phosphorylation of HSL and the lipid droplet coating protein perilipin. Phosphorylation of perilipin leads to its removal from the lipid droplet and migration of phosphorylated HSL from the cytosol to the lipid droplet where it catalyzes the hydrolysis of triglycerides (Wang M. et al., Chem. Biol., 2006, 13, 1019-1027).
Dysregulation of adipocyte lipolysis, resulting in elevated circulating non-esterified fatty acids (NEFA) is associated with obesity and co-morbidities including the development of type 2 diabetes (Unger R. H, Annu. Rev. Med. 2002, 53, 319-336). Obese or insulin resistant subjects have increased visceral adipose tissue depots. These depots contain elevated levels of HSL protein (Large, V. et al., 1998, J. Lipid. Res. 39, 1688-1695) and exhibit enhanced lipolytic activity as they are resistant to the insulin-mediated suppression of lipolysis. This results in increased plasma levels of free fatty acids, which further exacerbates insulin resistance due to the accumulation of triglycerides in tissues other than WAT such as liver, pancreas and muscle. The ectopic deposition of triglycerides results in pathological effects such as increased glucose production in the liver, decreased insulin secretion from the pancreas, and reduced glucose uptake and fatty acid oxidation in skeletal muscle. Thus, the elevated plasma levels of FFA due to increased HSL activity contributes to and worsens insulin resistance in obese and type 2 diabetic individuals. In addition, elevated FFA is related to increased production of the inflammatory cytokine TNF-alpha, by the adipose tissue (Hotamisigil, G. S., 1995, J. Clin. Invest. 95, 2409-2415). TNF-alpha further disrupts insulin signaling by the activation of serine kinases, such as JNK-1, which phosphorylated IRS-1 which depresses insulin signaling (Gao, Z. et. al., Mol Endocrinol, 2004, 18, 2024-2034). Thus, restoring the exaggerated plasma FFA and triglyceride levels through inhibition of HSL would reduce the accumulation of triglycerides in tissues other than WAT, such as liver, muscle and the pancreas resulting in decreased hepatic glucose output, increased muscle fatty acid oxidation and improving β-cell function. Inflammatory cytokine production would also be lessened, leading to further reductions in FFA production and improved insulin signaling. Elevated FFAs are also associated with increased cardiovascular risk, including atherosclerosis and myocardial dysfunction (Lopaschuk, et. al., Physiol Rev 2005, 85, 1093-129; Oliver, M F, QJM 2006, 99, 701-9) It has also been demonstrated that chronic low-dose lipid infusion in healthy patients induces markers of endothelial activation independent of its metabolic effects (Cusi, et. al., J. Cardiometab. Syndr. 2009, 3, 141-6). Here it was shown that modest lipid infusion elevates markers of endothelial activation-ET-1, ICAM-1, VCAM-1. Furthermore high lipolytic activity and elevated FFAs lead to increased insulin resistance and hypertension in hypertensive rats (Mauriege, et. al. J Physiol Biochem. 2009, 65, 33-41).
As HSL is a major hormone regulated lipase, it is known that during insulin resistant states, the ability of insulin to suppress lipolysis is reduced, and contributes to the increased FFA, ie. lipotoxicity. These fatty acids collect in the liver and lead to increased production of TAG, which are packaged into VLDLs which are secreted. There is also an accumulation of lipid in liver, leading to a fatty liver phenotype. Lipolysis is increased during diabetes and obesity which contributes to this phenotype. Therefore, reducing the activity of HSL would decrease the release of FFA to the blood, thus limiting the supply of FFA to the liver for TAG synthesis. Thus, HSL inhibitors could have beneficial effects as treatment of NAFLD (nonalkoholic fatty liver disease) and NASH (nonalkoholic steatohepatitis) (Jeffry R. Lewis et al, Dig Dis Sci 2010, 55: 560-578).